I imagine that Microsoft have their own code for handling the OpenType layout tables, given that they co-developed them. OpenType fonts come in two flavours, PostScript and TrueType, depending on which format the glyph outlines use. Microsoft only ships fonts with TrueType outlines. The PostScript outlines are in CFF format, which is an Adobe technology. I expect that the Adobe code is responsible for dealing with them, especially as there is no published standard describing how to rasterize PostScript fonts with pixel accuracy.